The present invention relates to adhesive compositions showing adhesive properties upon irradiation (with UV rays, for example) and adhesive sheets prepared therefrom.
Generally, one-pack solvent-free acrylic adhesive compositions that contain monomer as polymerization component and initiator, comprise a hard segment component showing a relatively high glass transition temperature (Tg) (e.g., about 60 to 180xc2x0 C.) and conferring high cohesion on adhesives (such as acrylic acid, methyl methacrylate, etc.), a soft segment component showing a relatively low glass transition temperature (eg, about xe2x88x9290 to xe2x88x9220xc2x0 C.) and conferring high adhesion on adhesives (such as butyl acrylate (Tg=xe2x88x9254 xc2x0 C.), 2-ethylhexyl acrylate (Tg=xe2x88x9285xc2x0 C.), etc.), a component showing an intermediate glass transition temperature (e.g., about xe2x88x9220 to 60xc2x0 C.) (such as t-butyl acrylate (Tg=41xc2x0 C.), vinyl acetate, etc.), and a photoinitiator. The acrylic adhesive composition is cured by irradiation to show adhesive properties (JPA No. 2000-073025, etc.).
However, adhesive compositions such as those described above have the disadvantage that, when the content of the soft segment component is increased to further improve adhesion, the content of the hard segment component relatively decreases to lower cohesion and on the contrary, when the content of the hard segment component is increased to further improve cohesion, the content of the soft segment component relatively decreases to lower adhesion. Thus, no adhesive compositions achieving high adhesion and high cohesion at the same time have been obtained.
One important application of solvent-free acrylic adhesive compositions is the use as adhesives for bonding a backing film to a flexible circuit board. However, conventional solvent-free acrylic adhesive compositions are insufficient in heat resistance against the lead-free solder technique that has recently come to be used in this field of application and has a melting point 30 to 40xc2x0 C. higher than that of traditional solder, and tackiness of adhesives is impaired during the solder reflow processes.
What is needed, therefore, is an adhesive composition that shows high adhesion and cohesion as well as good heat resistance.
In one aspect, the present invention relates to adhesive compositions containing a major component monomer having a glass transition temperature of xe2x88x9250xc2x0 C. or less when it is homopolymerized and an imide (meth)acrylate in a specific ratio that show high adhesion and high cohesion at the same time while maintaining heat resistance.
Accordingly, in one aspect the present invention provides an adhesive composition comprising an imide (meth)acrylate, a homopolymerizable monomer having a glass transition temperature of xe2x88x9250xc2x0 C. or less when it is homopolymerized, and a photoinitiator, wherein the content of the imide (meth)acrylate is 1 to 20 parts by weight per 100 parts by weight of the monomer.
In another aspect, the present invention provides an adhesive composition wherein the imide (meth)acrylate is 2-(perhydrophthalimide-N-yl)ethyl acrylate.
In another aspect, the present invention comprises an adhesive composition that includes 1 to 5 parts by weight of a compound per 100 parts by weight of the homopolymerizable monomer, and the compound is copolymerizable with the homopolymerizable monomer.
In one aspect, the present invention comprises an adhesive composition wherein the compound that is copolymerizable with homoplymerizalbe monomer includes at least one member selected from the group of acrylic acid, isobomyl acrylate and morpholine acrylate.
In one aspect, the present invention also provides an adhesive sheet comprising an adhesive layer where the adhesive layer is formed by curing an adhesive composition. The adhesive composition includes imide (meth)acrylate and homopolymerizable polymer and photoinitiator, wherein a glass transition temperature of the homopolymerizable monomer is xe2x88x9250xc2x0 C. or less when it is homopolymerized, and the content of the imide (meth) acrylate is 1 to 20 parts by weight per 100 parts by weight of the monomer.